1. Field of the Invention
The present invention relates to an engine exhaust gas cleaning system.
2. Description of the Prior Art
It is a well known technology in the art that an air-fuel ratio is set to a value excess in air concentration (hereinafter referred to as lean air-fuel ratio) more than the theoretical air-fuel ratio (hereinafter referred to as stoichiometric air-fuel ratio) to perform lean combustion in order to improve fuel economy of an engine.
For example, there are known a technology that lean combustion under an air-fuel ratio of 20 to 25 is attained in an engine of a type in which fuel is injected at a position near an intake port of an intake pipe portion (port injection type), and a technology that very lean combustion under an air-fuel ratio of 40 to 50 is attained by forming a stratified mixed gas in an engine of a type in which fuel is directly injected into a cylinder (in-cylinder injection type). In these technologies, fuel economy can be improved by lean combustion, that is, by increasing an amount of intake air to decrease pumping loss and heat loss.
In the case of stoichiometric combustion, exhaust gas can be cleaned by oxidizing and reducing HC, CO and NOx in the exhaust gas at a time using a three way catalyst. However, in the case of lean combustion, it is difficult to reduce NOx because the exhaust gas is in an oxygen excess state. In order to solve the problem, there is known an exhaust gas cleaning system for an engine. The exhaust gas cleaning system is that an NOx trapping agent which traps NOx in the exhaust gas by absorption or adsorption when the air-fuel ratio of the exhaust gas is lean and releases NOx to reduce or contact-reduce the NOx when the air-fuel ratio of the exhaust gas is rich (fuel excess) is disposed in the exhaust gas passage, and the air-fuel ratio of the exhaust gas is temporarily changed from a lean air-fuel ratio to the stoichiometric air-fuel ratio or a rich air-fuel ratio in an appropriate cycle to release or to reduce the NOx trapped to the NOx trapping agent in order to recover the NOx trapping ability (hereinafter, referred to as xe2x80x9cpurgexe2x80x9d).
In such an exhaust gas cleaning system, it is preferable from the viewpoint of the fuel economy and reduction of the exhaust gas components such as HC in the exhaust gas to temporarily change the air-fuel ratio to the stoichiometric air-fuel ratio or the rich air-fuel ratio only for a time period which is commensurate with an amount of the trapped NOx.
A technology for judging completion of release of NOx when the air-fuel ratio is temporarily changed to the stoichiometric air-fuel ratio or the rich air-fuel ratio is proposed in Japanese Patent No. 2692380 (WO94/17291). The completion of release of NOx after temporarily changing the air-fuel ratio from the lean air-fuel ratio to the stoichiometric air-fuel ratio or the rich air-fuel ratio is judged from the time when an air-fuel ratio detected by an air-fuel ratio sensor disposed downstream of an NOx trapping agent is switched from a lean state to a rich state. This is based on the fact that the air-fuel ratio detected by the air-fuel ratio sensor disposed downstream of the NOx absorbent is shown to be slightly lean because HC and CO in the exhaust gas are consumed in reduction of the NOx until the NOx absorbed to the NOx absorbent is released and reduced even if the air-fuel ratio upstream of the NOx absorbent becomes the stoichiometric air-fuel ratio or the rich air-fuel ratio, and consequently the air-fuel ratio detected by the air-fuel ratio sensor becomes rich after completion of release and reduction of the NOx absorbed to the NOx absorbent.
As a similar technology, Japanese Patent Application Laid-Open No. 10-128058 (U.S. Pat. No. 5,771,685) discloses a technology that the performance of an NOx trapping agent is monitored by estimating an amount of trapped NOx from a time interval between the time when the air-fuel ratio is switched from a lean air-fuel ratio to the stoichiometric air-fuel ratio or a rich air-fuel ratio and the time when the air-fuel ratio detected by the air-fuel ratio sensor disposed downstream of the NOx trapping agent is switched from a lean state to a rich state.
Further, Japanese Patent Application Laid-Open No. 8-260949 discloses a technology that the NOx absorbing performance and the oxygen storage capacity of the NOx absoebent are separated from each other because an output of the air-fuel ratio sensor is influenced by the NOx absorbent and the oxygen storage capacity possessed by a catalyst disposed at an upstream or downstream position close to the air-fuel ratio sensor when the performance of the NOx absorbent is monitored based on the air-fuel ratio detected by the air-fuel ratio sensor disposed at the position downstream of the NOx absorbent. In this technology, the oxygen storage capacity is detected by an output of the air-fuel ratio sensor disposed at the position downstream of the NOx absorbent when the amount of the absorbed NOx is nearly zero.
However, the output waveform of the air-fuel ratio sensor disposed downstream of the NOx trapping agent is affected an amount of the reducers such as HC, CO and so on flowing into the NOx trapping agent even if the amount of the NOx trapped to the NOx trapping agent is constant. Although the conventional technology discloses a method of estimating the amount of the reducers from an air-fuel ratio, the conventional technology does not take into consideration variations in controlled air-fuel ratio and what percentage of the reducers such as HC, CO and so on flowing into the NOx absoebent is actually used for reducing the absorbed NOx. Actually, the amount of the reducers exhausted from an engine is not totally used for the reduction of the NOx trapped to the NOx trapping agent, but part of the reducer is oxidized in the NOx trapping agent and in the catalyst arranged at a position upstream of the NOx trapping agent. Therefore, the percentage of the reducers used for the reduction of the NOx trapped to the NOx trapping agent to the total amount of the reducers exhausted from the engine is influenced by variations in operating condition and in the catalyst performance due to deterioration because the percentage differs depending on the performance of the NOx trapping agent and the catalyst performance (also influenced by the oxygen storage capacity) of the catalyst arranged upstream of the NOx trapping agent.
Further, because the performance of the NOx trapping agent is strongly influenced by temperature of the exhaust gas (temperature of the NOx trapping agent itself), the amount of trapped NOx is reduced, for example, when the exhaust gas temperature is too high even if the NOx itself is not deteriorated. Therefore, it may be erroneously judged that the NOx trapping agent is deteriorated. Furthermore, since the lean combustion is performed by delicate control of fuel injection timing and enhancement of intake air flow in the case of the in-cylinder injection type engine, the exhaust gas temperature sometimes fluctuates even if the engine is in a similar operating condition (rotating speed, load). However, the conventional technologies described above do not take the variations in the temperature into consideration. By mounting a temperature sensor and directly measuring the temperature of the NOx trapping agent to correct the temperature effect, the effect of the above-mentioned variations can be reduced. However, the cost is increased because it is necessary to use a comparatively highly accurate temperature sensor. In addition, it is also necessary to diagnose the temperature sensor itself, and accordingly the system becomes complex and high in cost.
Further, the output waveform of the air-fuel ratio sensor arranged downstream of the NOx trapping agent is influenced by the NOx trapping agent itself and/or the oxygen storage capacity of the catalysts closely arranged upstream and downstream of the NOx trapping agent even if the amount of NOx trapped to the NOx trapping agent is an equal value.
For example, if the NOx trapping agent itself has an oxygen storage capacity or the catalyst having the oxygen storage capacity is closely arranged upstream or downstream of the NOx trapping agent, oxygen is storaged in the NOx trapping agent or the catalyst during lean operation of the engine, and the storaged oxygen is released when the air-fuel ratio is switched from the lean air-fuel ratio to the stoichiometric air-fuel ratio or the rich air-fuel ratio. Therefore, the output of the air-fuel ratio sensor arranged downstream of the catalyst or the like having the oxygen storage capacity is influenced by the oxygen released from the catalyst or the like.
Therefore, when the trapped amount or the trapping performance of NOx trapped to the NOx trapping agent is estimated using the above-mentioned air-fuel ratio sensor, the oxygen storage capacity may become a large error factor. For example, if an amount of oxygen storage is large, the time period that the output of the air-fuel ratio sensor arranged downstream of the NOx trapping agent shows a lean state is lengthened when the air-fuel ratio is temporarily changed from the lean air-fuel ratio to the stoichiometric air-fuel ratio or the rich air-fuel ratio. Therefore, an amount of the trapped NOx is erroneously judged to be larger. On the other hand, if an amount of oxygen storage is small, the output of the air-fuel ratio sensor arranged downstream of the NOx trapping agent shows a rich state early when the air-fuel ratio is temporarily changed from the lean air-fuel ratio to the stoichiometric air-fuel ratio or the rich air-fuel ratio. Therefore, an amount of the trapped NOx is erroneously judged to be smaller.
Although the amount of oxygen storage reaches the oxygen storage capacity in a short time by performing lean operation, the oxygen storage capacity itself varies due to deterioration or the like to cause the erroneous judgment of NOx trapped amount described above.
The conventional technology disclosed in Japanese Patent Application Laid-Open No. 8-260949 requires a process that the amount of the NOx absorbed is brought to nearly zero in order to detect the oxygen storage capacity. Therefore, the operating condition at detecting the oxygen storage capacity is not always equal to the operating condition at detecting the amount of the NOx absorbed, and accordingly the oxygen storage capacity varies depending on the exhaust temperature difference. The detection error in the oxygen storage capacity is further increased by the variations of the air-fuel ratio, as described above.
An object of the present invention is to provide an engine exhaust gas cleaning system which can precisely detect NOx trapping performance of an NOx trapping agent by reducing the effects of variations and detection errors without high costs.
In order to solve the above-described problem, the present invention is characterized by an engine exhaust gas cleaning system comprising an NOx trapping agent arranged inside an exhaust gas passage of an engine, the NOx trapping agent trapping NOx in an exhaust gas by adsorbing or absorbing when an air-fuel ratio of the exhaust gas flowing thereinto is a lean air-fuel ratio, the NOx trapping agent purging by releasing and/or reducing the trapped NOx when a concentration of oxygen in the exhaust gas is reduced; and an air-fuel ratio changing means for temporarily changing the air-fuel ratio of the exhaust gas from a lean air-fuel ratio to a stoichiometric air-fuel ratio or a rich air-fuel ratio with a preset period, which further comprises a concentration detecting means for detecting a concentration of a specified component in the exhaust gas at a position downstream of the NOx trapping agent in the exhaust gas passage; an operating control parameter changing means for changing operating control parameters of the engine in order to change an amount of NOx trapped to the NOx trapping agent; and an NOx trapping amount change detecting means for detecting change in the amount of NOx trapped to the NOx trapping agent based on change in a detected result of the concentration detecting means at the time of the air-fuel ratio changing means temporarily changing the air-fuel ratio of the exhaust gas to the stoichiometric air-fuel ratio or the rich air-fuel ratio when the operating control parameter changing means changes the operating control parameters to change the amount of trapped NOx.
The engine exhaust gas cleaning system of the present invention is preferably characterized by that the operating control parameter changing means changes a time period in which the air-fuel ratio of the exhaust gas is lean.
The engine exhaust gas cleaning system of the present invention is preferably characterized by that the operating control parameter changing means changes a releasing amount of NOx from the engine during a time period in which the air-fuel ratio of the exhaust gas is lean.
The engine exhaust gas cleaning system of the present invention is preferably characterized by that the operating control parameter changing means changes at least any one of ignition timing, fuel injection timing, an EGR ratio and an air-fuel ratio in order to change a releasing amount of NOx from the engine during a time period in which the air-fuel ratio of the exhaust gas is lean.
The engine exhaust gas cleaning system of the present invention is preferably characterized by that the concentration detecting means measures any one of oxygen concentration, NOx concentration, HC concentration and Co concentration in the exhaust gas.
The engine exhaust gas cleaning system of the present invention is preferably characterized by that the operating control parameter changing means changes the operating parameter in order to increase or decrease a predetermined NOx trapping amount, and the engine exhaust gas cleaning system further comprises an NOx trapping agent performance judging means for judging NOx trapping performance of the NOx trapping agent based on a detecting result of the NOx trapping amount change detecting means.
The engine exhaust gas cleaning system of the present invention is preferably characterized by that the NOx trapping agent performance judging means judges that the NOx trapping performance of the NOx trapping agent is deteriorated when a detecting result of the NOx trapping amount change detecting means is not larger than a predetermined value.
The engine exhaust gas cleaning system of the present invention is preferably characterized by that the operating control parameter changing means changes the operating parameter in order to increase or decrease a predetermined NOx trapping amount, and the engine exhaust gas cleaning system further comprises a lean operating time period changing means for changing a time period in which the air-fuel ratio of the exhaust gas is lean based on a detected result of the NOx trapping amount change detecting means.
The engine exhaust gas cleaning system of the present invention is preferably characterized by that the engine exhaust gas cleaning system comprises an NOx trapping performance judging means for judging that the NOx trapping performance of the NOx trapping agent is deteriorated when a lean operating time period changed by the lean operating time period changing means becomes smaller than a predetermined value.
The engine exhaust gas cleaning system of the present invention is preferably characterized by that the engine exhaust gas cleaning system comprises a lean operation limiting means for limiting lean operation when the NOx trapping performance judging means judges that the NOx trapping agent is deteriorated.
The engine exhaust gas cleaning system of the present invention is preferably characterized by that the engine exhaust gas cleaning system memorizes a code expressing deterioration of the NOx trapping agent and/or generates a warning when the NOx trapping performance judging means judges that the NOx trapping agent is deteriorated.
According to the present invention, when the operating parameter is changed in order to change the amount of trapped NOx of the NOx trapping agent, the change in the amount of trapped NOx of the NOx trapping agent is detected from the detected result of the concentration detecting means. Therefore, it is possible to provide an engine exhaust gas cleaning system which can accurately detect the change in the amount of trapped NOx without being affected by various kinds of factors.